Add ability to order / prioritise testitems?

[nickrobinson251]

Ideally testitems would be independent, but if for some reason they are not (#211) you may want certain testitems to run first/last. Currently, we don't give users control over the order testitems run in (even if there's not multiple workers, the order isn't guaranteed).

I'm thinking it'd be useful to give users the ability to assign an order/priority to testitems, e.g. based on file, name and/or tag. If we had integer "priorities", then when selecting testitems to run, all testitems with a higher priority must be run/running before any of a lower priority can run.

We'd have to figure out how this would interact with ordering based on the last test status (#172) and any ability to assign testitems to workers (#47)

I'm not sure we want this ability at all. I don't believe e.g. go test has this.